The most common process for the preparation of carboxylic acid esters is the direct reaction of the acids with alcohols. It leads to an equilibrium in which alcohol, acid, ester and water are all present. The adjustment of the equilibrium is affected by catalysts.
This procedure is generally economic only when the esterification equilibrium is shifted as far as possible towards the formation of esters. Suitable measures of achieving this aim are the use of a high excess of one of the starting materials or the removal of one of the products from the reaction mixture. Normally, the alcohol is added in excess, or the reaction water or the ester is removed from the reaction mixture.
The reaction water can, for example, be separated by the addition of a dehydrating agent (which binds the water), or preferably by azeotropic distillation with the aid of separating agents. With the extractive esterification process, the ester formed is selectively dissolved out of the reaction mixture with the aid of a solvent.
However, the removal of the reaction water by azeotropic distillation is not suitable for the preparation of methyl esters of carboxylic acids, since methanol distills with the separating agent in such amounts that no phase separation takes place.
According to a process described in J. Org. Chem. 24, 261 (1959), methyl esters of carboxylic acid are obtained by the reaction of carboxylic acids with methanol in the presence of acetone dimethyl acetal, the dimethyl acetal acting as a hydrophilic agent and supplier of methanol. If adipic acid, methanol and acetone dimethyl acetal are reacted in a molar ratio of 4:5:8, a 94% yield of dimethyladipate is obtained. The economy of this procedure is impaired by the fact that the use of acetone dimethyl acetal increases the raw material costs, and the formation of acetone during the course of the reaction increases the amount of distillation required.
According to the process of Clinton and Laskowski (J. Amer. Chem. Soc. 70, 3135 (1948)), in order to prepare methyl esters, a carboxylic acid and methanol are heated under reflux in the presence of sulfuric acid as a catalyst and methylene or ethylene chloride without the reaction water being removed. Then the organic phase containing the ester is separated and recovered. This procedure not only requires the use of chlorinated hydrocarbons as solvents for the ester, but also necessitates long reaction periods of 6 to 15 hours.
DE-OS No. 20 50 678 describes the preparation of esters for carboxylic acids and alcohols in the presence of inert organic solvents and esterification catalysts. Hydrocarbons having a boiling point of -10.degree. to 200.degree. C. are used as the solvents, and acids--or materials forming acids in the aqueous medium--are used as catalysts together with aqueous saline solutions. According to the examples, the preparation of methyl esters of saturated carboxylic acids requires reaction periods of 10 hours (propionic acid) to 60 hours (valeric acid), the ester yields being only 68% and 66%, respectively. The long reaction periods, the use of hydrocarbons as extraction agents, ant the unsatisfactory speak against the use of this process variant.
Other processes for obtaining methyl carboxylates were developed more for use in the laboratory than for use on an industrial scale. This includes esterification with diazo methane or with BF.sub.3 -methanol reagent. The use of molecular sieves (Harrison et al., Chemistry and Industry (1968), page 1568) for adsorbing the reaction water has not been adopted on an industrial scale.